The present invention relates to a semiconductor device including a ferroelectric film or a dielectric film with a high dielectric constant (which will be herein referred to as a xe2x80x9chigh-dielectric-constant filmxe2x80x9d) and to a method for fabricating the device.
Recently, nonvolatile or large-capacity semiconductor memories have been developed using a ferroelectric material or a dielectric material with a high dielectric constant. Each of these dielectric materials is made by sintering a metal oxide and contains a lot of easily reactive oxygen. Accordingly, when a capacitor, including a capacitive insulating film of such a dielectric material, is formed, the upper and lower electrodes of the capacitor, located over and under the capacitive insulating film, respectively, must be made of some material showing sufficient stability against the oxidation. Examples of the applicable materials include an alloy mainly composed of platinum.
A known semiconductor device includes a passivation film on the uppermost surface thereof. The passivation film is deposited over the structure already including a capacitor and an interlevel dielectric film, and is typically made of silicon nitride or silicon dioxide. Normally, the passivation film is formed by a CVD (chemical vapor deposition) process, and often contains hydrogen or moisture therein.
Also, when a semiconductor device and its associated members are molded together with a resin encapsulant by a transfer molding process, for example, the resin encapsulant used for the process often contains some filler (which is usually silica). However, the particles of the filler have a high hardness, thus possibly doing serious damage on the surface of the device during the resin molding process. In addition, in fabricating a DRAM (dynamic random access memory), an xcex1-ray is emitted from the radioactive components of the filler and sometimes causes soft errors in the memory.
Therefore, to prevent the surface of a semiconductor device from being damaged by the filler particles or to shield the device from the xcex1-rays emitted therefrom, the surface is often covered with a coating of some organic material (e.g., polyimide). Also, the surface of a device is sometimes given double protection. Specifically, a passivation film of an inorganic insulator is deposited over the surface first, and then a surface coating of polyimide is formed on the passivation film. The polyimide surface coating is normally formed by heating and curing a film of a polyimide precursor composition at a temperature of about 350-450xc2x0 C.
Accordingly, a semiconductor device including a ferroelectric or high-dielectric-constant film also needs to have its surface covered with a polyimide coating because of similar reasons. In the current state of the art, however, where a polyimide coating is formed on the surface of a semiconductor device including a capacitive insulating film made of a ferroelectric material, the polarization properties of the ferroelectric film should degrade while the polyimide is heated to form the coating. Therefore, the polyimide coating is hard to apply to the actual fabrication process of such a device. This is because while the polyimide precursor is being heated and cured, hydrogen or moisture, contained in the passivation or interlevel dielectric film of the device, adversely diffuses into the ferroelectric film due to the heat, thus degrading the polarization properties of the ferroelectric film.
The degradation is believed to occur through one of the following mechanisms. One possibility is that platinum, contained in the upper and lower electrodes, may react with hydrogen and act as a catalyst that reduces the material of the ferroelectric film (i.e., an oxide). Another possibility is that the moisture reacts with the material of metal interconnects made of aluminum, for example, to produce hydrogen and thereby degrade the polarization properties of the capacitor (see The Institute of Electronics, Information and Communication Engineers Transactions, C Vol. J83-G No. 1, pp.53-59).
To solve this problem, a countermeasure process was proposed in Japanese Laid-Open Publication No. 10-270611, for example. In the proposed process, a polyimide film is formed as a surface coating for a semiconductor device including a ferroelectric film by heating and curing a film of a polyimide precursor composition at a temperature of 230-300xc2x0 C. According to this method, the polarization properties of the ferroelectric film do not degrade so much. It should be noted that the same problem might occur in the high-dielectric-constant film as well as in the ferroelectric film.
Hereinafter, a known semiconductor device and a method for fabricating the device will be described with reference to FIGS. 6 through 7C. As an exemplary known semiconductor device, FIG. 6 schematically illustrates a cross-sectional structure for one of the one-transistor, one-capacitor memory cells of a ferroelectric memory.
The semiconductor device shown in FIG. 6 includes an MOS transistor 2 and a ferroelectric capacitor 3 that have been formed over a substrate 1. A surface coating 62 of polyimide has been formed to cover an interconnection layer 5 and a second insulating film 42 that are located over the transistor 2 and capacitor 3.
The MOS transistor 2 shown in FIG. 6 is made up of known components including source/drain regions and a polysilicon gate. In the illustrated example, the MOS transistor 2 includes gate electrode 21 of polysilicon, gate oxide film 22, sidewall 23, silicon nitride film 24, source/drain regions (doped regions) 25 and LOCOS 26.
The ferroelectric capacitor 3 is made up of lower electrode 32, upper electrode 34 and ferroelectric film 33 interposed between these electrodes 32 and 34. If necessary, an electrode contact layer 31 is additionally formed under the lower electrode 32. The ferroelectric film 33 may be made of any arbitrary material such as lead zirconate titanate (Pb(Zr,Ti)O3 (PZT)) or strontium bismuth tantalate (SrBi2Ta2O9 (SBT)).
A first insulating film 41 may be a silicon dioxide film or a silicon nitride film. In the former case, the first insulating film 41 may be a BPSG (borophosphosilicate glass), PSG (phosphosilicate glass) or O3-TEOS (tetraethylortho silicate) film, for example. The second insulating film 42 on the first insulating film 41 may be a silicon dioxide film formed by an APCVD (atmospheric-pressure chemical vapor deposition) process, for example. An interconnection layer 5 has been formed on the second insulating layer 42 and electrically connected to the MOS transistor 2 and ferroelectric capacitor 3.
Hereinafter, a method for fabricating the semiconductor device shown in FIG. 6 will be described with reference to FIGS. 7A through 7C. FIGS. 7A through 7C are cross-sectional views illustrating respective process steps for fabricating the known semiconductor device.
First, a semiconductor substrate 1 (which is preferably a wafer in the actual fabrication process) is prepared as shown in FIG. 7A. Next, MOS transistor 2, ferroelectric capacitor 3 and so on are formed by a known process on each active region, and then an interconnection layer 5 is formed thereon as shown in FIG. 7B.
Next, as shown in FIG. 7C, a surface coating 62 of polyimide, having a plurality of openings (not shown) over bonding pad regions, is formed to cover the substrate 1 that already includes the transistor 2, capacitor 3 and interconnection layer 5 thereon.
The surface coating 62 may be formed as follows. First, a photosensitive polyimide material, containing a polyimide precursor composition that will cure when heated to a temperature of 230-300xc2x0 C., is applied onto the surface of the substrate 1 that already includes the transistor 2, capacitor 3 and interconnection layer 5 thereon. Next, the film of the polyimide precursor composition is exposed to a radiation while being masked with a predetermined pattern. Subsequently, after the non-exposed parts of the film have been dissolved in a developer and removed, the remaining parts of the film are heated and cured at a temperature of 230-300xc2x0 C. Thereafter, the wafer 1 with the surface-coating 62 is diced into multiple semiconductor chips. Then, each of the chips and its associated members are molded together with a resin encapsulant to obtain a package, which is then subjected to an assembling process to mount the package onto a circuit board.
In this manner, a semiconductor device, including a ferroelectric capacitor and a surface coating of polyimide, is fabricated. In this known technique, the polyimide precursor is heated and cured at 230-300xc2x0 C., i.e., a temperature much lower than the heat treatment temperature of well over 300xc2x0 C. at which serious degradation in polarization properties of the ferroelectric film is observed. Thus, this technique can suppress the degradation to a certain degree.
However, the present inventors found out that the known technique can not sufficiently suppress the degradation of the ferroelectric film 33 in the semiconductor device. The ferroelectric film 33 of the known semiconductor device still degrades even though the heat treatment temperature of the polyimide film is much lower than the normal one of about 350-450xc2x0 C. The reason is probably as follows. Specifically, a film included in the semiconductor device may emit some gas and produce water molecules even at 300xc2x0 C. or less, thus degrading the ferroelectric film 33.
If the degradation of the ferroelectric film 33 can not be suppressed sufficiently, then each device of a large-capacity or densely integrated circuit is affected by the degradation particularly seriously. In the circuit of that type, the ferroelectric (or high-dielectric-constant) film 33 thereof also has a very small size. Accordingly, when the ferroelectric film 33 degrades, each of the devices (e.g., capacitors) included in the circuit is affected by the degradation much more greatly. For that reason, the semiconductor device as a whole also has its performance deteriorated seriously by the degradation of the ferroelectric film 33. For example, suppose the semiconductor device is a memory with a storage capacity of several megabits. In that case, should some memory cells, representing just several bits, fail due to the degradation caused by any variation in the fabrication process thereof, the yield of the semiconductor devices decreases considerably, thus making it difficult to manufacture the devices stably enough.
It is therefore an object of the present invention to provide (1) a highly reliable semiconductor device, including a ferroelectric or high-dielectric-constant film, without degrading its performance and (2) a method for fabricating a device of that type.
A semiconductor device according to the present invention includes at least a ferroelectric or high-dielectric-constant film and a surface coating that have been stacked in this order over a substrate. In this device, the surface coating is made of an acrylic resin.
Another semiconductor device according to the present invention includes at least a ferroelectric or high-dielectric-constant film and a surface coating that have been stacked in this order over a substrate. In this device, the surface coating is made up of multiple layers, at least one of which is made of an acrylic resin.
In one embodiment of the present invention, the ferroelectric or high-dielectric-constant film may be a capacitive insulating film for a capacitor.
Still another semiconductor device according to the present invention includes: a capacitor including a ferroelectric or high-dielectric-constant film as a capacitive insulating film; an insulating film that covers the capacitor; and a surface coating that covers the insulating film. In this device, the surface coating is made of an acrylic resin.
An inventive method for fabricating a semiconductor device includes the steps of: forming a ferroelectric or high-dielectric-constant film over a substrate; depositing an acrylic resin over the substrate to cover the ferroelectric or high-dielectric-constant film with the acrylic resin; and heating and curing the acrylic resin.
Another inventive method for fabricating a semiconductor device includes the steps of: forming a capacitor, which includes a ferroelectric or high-dielectric-constant film as a capacitive insulating film, over a substrate; depositing an acrylic resin over the substrate to cover the capacitor with the acrylic resin; and heating and curing the acrylic resin.
Still another inventive method for fabricating a semiconductor device includes the steps of: forming a capacitor, which includes a ferroelectric or high-dielectric-constant film as a capacitive insulating film, over a substrate; forming an insulating film over the substrate to cover the capacitor with the insulating film; depositing an acrylic resin over the insulating film; and heating and curing the acrylic resin.
In one embodiment of the present invention, the step of heating and curing the acrylic resin may include heating the acrylic resin to a temperature of 250xc2x0 C. or less.
In this particular embodiment, the acrylic resin is preferably heated by a hot plate.
According to the present invention, the surface coating is made of an acrylic resin, which can be heated and cured at a lower temperature than a polyimide resin as a material for the known surface coating. Thus, even in the heat, it is still possible to minimize the unwanted diffusion of hydrogen or moisture from some film of the semiconductor device into the ferroelectric or high-dielectric-constant film thereof. As a result, it is possible to suppress the degradation in performance of the semiconductor device. Also, according to the present invention, a hot plate is used to heat and cure the acrylic resin. Thus, the acrylic resin can be heated and cured in a shorter time than any other heat treatment using some furnace such as an oven or diffusion furnace.